Bugfix/harden activity timeout (#4545)

* add some hardening

* add info memory logging

* fix last_observed

* remove log spam

* properly cache last activity details

* default values

---------

Co-authored-by: Richard Kuo (Onyx) <rkuo@onyx.app>

backend/onyx/redis/redis_connector_index.py

@@ -165,6 +165,16 @@ class RedisConnectorIndex:
 
         return False
 
+    def connector_active_ttl(self) -> int:
+        """Refer to https://redis.io/docs/latest/commands/ttl/
+
+        -2 means the key does not exist
+        -1 means the key exists but has no associated expire
+        Otherwise, returns the actual TTL of the key
+        """
+        ttl = cast(int, self.redis.ttl(self.connector_active_key))
+        return ttl
+
     def generator_locked(self) -> bool:
         return bool(self.redis.exists(self.generator_lock_key))